Governments are increasingly regulating the exhaust emissions of vehicles. In particular, vehicles powered by diesel engines must meet more and more stringent regulations in the next several years. Cellular ceramic filters are used in high temperature engine exhaust streams as particle traps intended to satisfy certain regulations. These trap filters over time become loaded with soot. To avoid unacceptable back pressures from developing, these filters must be periodically regenerated to restore performance to that of a clean, or nearly clean, filter. The regeneration process requires control of several variables in order to achieve long trap life. A critical variable is the amount of particle mass on the trap before regeneration. So that trap life can be predictable, this particle mass just before regeneration must be approximately constant for each regeneration. Measurement of captured mass directly, however, is a difficult endeavor. The trap cannot be weighed during usage. The only typical quantities sensed are: engine air flows and temperatures, exhaust back pressure and trap differential pressure, trap temperature, and engine speed and torque.
Known regeneration systems tend to initiate when the trap differential pressure reaches a predetermined threshold value, e.g., U.S. Pat. No. 4,608,640, or a ratio of various pressures, e.g., U.S. Pat. Nos. 4,610,138 and 4,630,438. In spite of these systems which rely on pressures and differential pressures, it is well known that the trap differential pressure increases with the amount of particle loading, but at different engine speeds and loads, the pressure drop varies substantially. Furthermore, it is known that the range of exhaust temperatures affect the pressure drop to flow relationship. That is, for a given trap loading, an engine idle condition creates a pressure drop, while an engine speed several times faster than idle creates a substantially increased pressure drop. Whereas regeneration might not be triggered by the pressure drop at low engine speed, it very well may be triggered at the higher speed. Thus, depending on engine speed, the loading of the trap could vary substantially from one regeneration to the next. Such variability leads to several problems, including excessive regeneration and shorter filter life, possible overloading of the engine, etc. The present invention solves that problem and provides for an apparatus and method which regenerates at a relatively constant trapped mass weight time after time.